Triggered by seasonal changes in available sunlight, about
50 million square kilometers of the Earth’s land surface undergo a
transition from freeze to thaw each year. Snow and ice disappear then
return, frozen ground softens and hardens, vegetation “greens
up” and fades. Satellite sensors can detect many of these processes
through seasonal changes in the amount of sunlight reflected by the
Earth’s surface at various wavelengths. The percentage of light that a
surface reflects out of the total light falling on it is the surface’s
albedo. Bright, reflective surfaces, such as fresh snow, have a high
albedo; dark, absorptive surfaces, such as dense forest, have a low
albedo.

These globes display seasonal albedo observations over much of Asia
collected by NASA’s Multi-angle Imaging SpectroRadiometer (MISR) in
2004. The left-hand column of globes is natural-color, made by combining
MISR-observed albedos in a way that is similar to what our eyes would see.
The right-hand set of globes shows the albedo for only the wavelengths of
light that plants use for photosynthesis or the albedo for
Photosynthetically Active Radiation (PAR). Places where the surface is
reflecting most of the photosynthetic wavelengths (high albedo) are
off-white or tan; places where the surface is absorbing most of the
photosynthetic wavelengths (low albedo) are dark green to nearly black. Gray
areas indicate lack of data, for example, because of persistent cloudiness.
Particles in the atmosphere can interfere with the collection of
surface-albedo measurements because the particles scatter incoming sunlight
in all directions; however, all the scattering effects from the atmosphere
have been removed from these albedo globes.

In December through February, snow covers the northern latitudes, making
the land surface more reflective, and therefore increasing the albedo. The
albedo in photosynthetically active wavelengths is also high (white, tan,
and yellow values) because it is winter, and plants are not very active. As
the seasons progress, the albedo in photosynthetically active wavelengths of
light decreases because plants are abundant and active; by the
June-through-August period, vegetation across Asia was absorbing almost all
of the photosynthetic light coming in from the Sun. Areas with higher
albedos even during the summer growing season include the semi-arid Tibetan
Plateau (roughly centered in the globes), the Arabian Peninsula (far left),
and the high Arctic (top).

The Multi-angle Imaging SpectroRadiometer observes the daylit Earth
continuously from pole to pole, and every 9 days views the entire globe
between 82 degrees north and 82 degrees south latitude. MISR was built and
is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA's
Office of Earth Science, Washington, DC. The Terra satellite is managed by
NASA's Goddard Space Flight Center, Greenbelt, MD. JPL is a division of the
California Institute of Technology.